1. Field of the Invention
The present invention relates to a detection apparatus and a method for detecting whether light is interrupted.
2. Description of the Related Art
Conventionally, the reference position in a machine that drives mechanism therein is detected by mechanical switches or photo-sensors.
However, the mechanical switches include tolerance and backlash and therefore they are not suitable for detecting the reference position with high accuracy. In detecting the reference position using a photo-sensor, generally, a current (light emission amount) on the light emitting side is fixed at a predetermined level, and an output from the light-receiving side is detected by comparing with a threshold value as the reference position.
In this case, a light emission amount may be set to be large on the light-emitting side and a load resistance value may be set to be large on the light-receiving side, so that the output of the light-receiving side reaches a saturation level even though a very small current induced by the incident light flows on the light-receiving side.
With regard to the encoder, Japanese Patent Application Laid-Open No. 08-145722 discusses a technique that a current from the LED is controlled so that added up current of output currents from a plurality of sensors should be at constant level to prevent drop in detection sensitivity due to dark current. Regarding a threshold value for making a decision in detecting a position, U.S. Pat. No. 6,079,892 discusses a detection apparatus for detecting an edge of paper by reflected light.
In the conventional methods, however, the detected position will vary owing to factors, such as a light emission amount of the light-emitting-side LED in a photo-sensor, photoelectric conversion characteristics of a light-receiving sensor, signal-to noise characteristics of an output signal, and threshold value characteristics for digitizing an output signal.
More specifically, the output level of the photo-sensor changes with variation of the photo-sensor, and also due to the effects of environmental change, such as temperature and voltage, and deterioration with time. Therefore, a resulting problem is that the reference position to be detected changes, and thus it is impossible to detect the reference position with high accuracy.
This problem will be concretely described with reference to FIG. 7. FIG. 7 illustrates relationship between mechanical positions and output levels when the photo interrupter is used as a photo-sensor.
In FIG. 7, the horizontal axis indicates the position of a reference position detection flag (light blocking plate). The vertical axis indicates the output level of the light-receiving sensor. The dotted line represents the width of the slit opening of the photo interrupter with respect to the positions of the reference position detection flag on the horizontal axis.
In FIG. 7, depending on the position of the reference position detection flag, the operation of the photo interrupter is illustrated in three states: a light-blocked state in which the light of the photo interrupter is completely blocked, a partially light-blocked state, and an open state in which all of the light reaches the light-receiving unit.
In the completely light-blocked state in which the reference position detection flag completely covers the slit opening of the photo interrupter, because the emitted light does not reach the light-receiving unit, the output level is at a level close to “0” (only a dark current flows). On the other hand, in the open state in which the reference position detection flag is positioned outside of the slit opening of the photo interrupter, the output level of the light-receiving sensor is at a high, almost saturated level.
Conventionally, the reference position is set to be at a position where an intermediate value between the output levels of the light-receiving sensor respectively obtained in the light-blocked state and in the open state. However, in this case, the reference position changes by the factors described above. In other words, by variations of the sensors, changes in the light amount emitted from the LED, and temperature changes, for example, the output level of the light-receiving sensor changes as indicated by the solid line, the dotted line, and the long and short dash line in FIG. 7, and therefore a detected reference position may vary.